Low reflectivity broadband fiber Bragg gratings are used for WDM/TDM hybrid multiplexed acoustic sensor systems due to their capability of being made for large scale multiplexing in single fiber. A single FBG is easy to characterize in terms of its peak reflectivity and spectral profile using a broadband source and an optical spectrum analyzer. For a multiplexed sensor cable, cable designers would prefer avoiding splicing due to the inconvenience and increased loss that is harmful for sensor array performance, and potentially weakened tensile strength.
To write many FBG's in series for continuous length of single fiber monitoring each FBG's reflectivity profile, and to evaluate the array during/after sensor cable fabrication using the fiber, and even after field deployment of the sensor cable, a proper device and procedure is desired to evaluate individual FBGs in the array.
For an evaluation of an array, a setup with a high coherent scanning laser and spectrum analyzer has been used with a reference reflector which is usually a Fresnel reflector of ˜3.4% reflectivity. Acquired data is usually quite noisy for a variety of reasons, and the evaluation results, even with averaged data, can be unreliable. With all the best effort, even the reference Fresnel reflections showed ripples, which supposed to have flat top over wavelength variations. It is suspected that the ripples are contributed by interferometric noise due to the high coherent source with multiple parasite reflections in the system.
There is a growing need to develop a broadband (at least tens of nm wide) pulsed source for some applications. One is for a multiplexed VIP MOMS sensor system, and another is multiplexed EFPI sensor demodulator system.
If a broadband pulsed source is used for an FBG reflector array evaluation system, the evaluation speed should be potentially much faster without needing the scanning of laser, and the evaluation reliability should be increased due to more stable data without the interferometric noises due to the low coherent broadband source even with the presence of multiple parasite reflections in the system.
This disclosure presents a system configuration and procedure that could be used during an FBG reflector array manufacturing, as well as field evaluations.